Condition: at room temperature.

In order to give an intuitionistic understanding about the droplets and micro-structures of microemulsions in the SFME system, cryogenic scanning electron microscopy (Cryo-SEM) was performed according to previous reports,^{39, 40} and conducted on the above microemulsion systems (points c, k and j in Figure 3) dissolving with or without lignin from different subregions. It is clear to see the existence of the discrete spherical droplets in the O/W microemulsion system containing lignin (point c in Figure 3), where the average diameter of the droplets is about 67 nm (Figure 5A and Figure S3A). Figure 5B shows the micro-structure of B.C microemulsion (point k in Figure 3) in presence of lignin. The obvious sponge-like structures, which are generated owing to the break of microemulsion droplets alongside the interconnections between two phases, are considered to be the feature structure of B.C microemulsion. The Cryo-SEM image of W/O microemulsion (point j in Figure 3) with lignin is also exhibited (Figure 5C) and the average diameter of its droplets is around 50 nm (Figure S3B). These Cryo-SEM images verify visually that the O/W, B.C and W/O microemulsions are formed orderly with the increase of octane content (W_O) when the R_P/W is fixed at 4.78. Furthermore, the above O/W microemulsion without lignin was also compared (Figure 5D). Surprisingly, the size of its droplets becomes to the smallest one with an average diameter of 48 nm (Figure S3C).